The invention relates to additive formulations for improving the transport of oilfield effluents in which hydrates of natural gas, petroleum gas or other gases can form. It also relates to a process using such a formulation.
Gases which form hydrates can comprise at least one hydrocarbon selected from methane, ethane, ethylene, propane, propene, n-butane and iso-butane, and possibly H2S and/or CO2.
Such hydrates form when water is found in the presence of gas, either in a free state or dissolved in a liquid phase such as a liquid hydrocarbon, and when the temperature reached by the mixture, in particular water, gas and possibly liquid hydrocarbons such as oil, drops below the thermodynamic hydrate formation temperature, that temperature being given for a known composition of gases at a fixed pressure.
Hydrate formation is feared in particular in the gas and oilwell industry where hydrate formation conditions can be satisfied. In order to reduce the production costs of crude oil and gas, both as regards investment and exploitation, one route, particularly for offshore production, is to reduce or even do away with treatments carried out on the crude or the gas to be transported from the field to the coast and in particular to leave all or part of the water in the fluid to be transported. Offshore treatments are generally carried out on a platform located on the surface near the field, such that the effluent, which is initially hot, can be treated before the thermodynamic hydrate formation conditions are satisfied when seawater cools the effluent.
However in practice, when the thermodynamic conditions required for hydrate formation are satisfied, hydrate agglomeration causes the transport conduits to block by the formation of plugs which prevent the passage of any crude oil or gas.
Hydrate plug formation can cause a production stoppage and thus result in substantial financial losses. Further, restarting the installation, especially when it involves offshore production or transport, can be a long process, as it is difficult to decompose the hydrates which have formed. When the production from an undersea natural gas or crude oil and gas field comprising water reaches the surface of the sea bed and is then transported along the sea bottom, the reduction in the temperature of the effluent produced can mean that the thermodynamic conditions for hydrates to form are satisfied and they form, agglomerate and block the transfer conduits. The sea bottom temperature can, for example, be 3xc2x0 C. or 4xc2x0 C.
Favorable conditions for hydrate formation can also be satisfied onshore when conduits are not buried (or are not buried deeply) in the soil, for example when the ambient air temperature is sufficiently low.
In order to overcome these disadvantages, prior authors have sought products which when added to a fluid can act as inhibitors by reducing the thermodynamic hydrate formation temperature. They are mainly alcohols, such as methanol, or glycols such as mono-, di- or tri-ethylene glycol. However, to be efficient, such products must be used in large quantities (10% to 40% of the amount of water), bringing with it two major disadvantages: high production costs and major pollution problems.
Insulation of the transport conduits has also been recommended, to prevent the temperature of the transported fluid from reaching the hydrate formation temperature under the operating conditions. However, this technique is also very expensive.
Two products based on using low quantities of additives (less than 1% by weight with respect to the water) have also been proposed.
The first process uses surfactants, preferably polymers, which disperse the hydrates to transport the hydrates in the form of a suspension in the effluent under consideration. Such a process has been described widely and in particular by the Applicant in previous patents, for example in European Patents Nos. 323 774 and 323 775.
The second process is based on adding water-soluble additives (known as kinetic inhibitors) which can modify hydrate crystallization (germination/growth/agglomeration) so that the transport time for effluents in the conduits is less than the time preceding hydrate formation or the time taken by the hydrates to grow and/or agglomerate to form a plug. Prior art kinetic inhibitors used in such a process are water-soluble polymers or copolymers which substantially prolong the hydrate formation time and substantially reduce the growth rate of the hydrate crystals. Such kinetic inhibitors, used alone or as a mixture, have been described in International Patent applications WO 94/12761, 96/04462, 96/29501 and 97/13824; and U.S. Pat. Nos. 5,723,524 and 5,744,665.
While they can substantially retard hydrate formation, such additives nevertheless suffer from the enormous disadvantage of leading to uncontrolled hydrate formation when the limits of use of such additives are breached.
We have now discovered a water-soluble additive formulation which can improve the transport of oilfield effluents comprising water, a gas and possibly a liquid hydrocarbon phase under thermodynamic conditions which are likely to lead to hydrate formation, this additive formulation being characterized in that it can better control hydrate formation by creating a very large number of small crystals and avoiding the disadvantage cited above and difficult to foresee from prior art kinetic inhibitor formulations.
Thus the present invention provides an additive formulation and a process for improving hydrate transport in a fluid comprising water, a gas and possibly a liquid hydrocarbon phase, under conditions in which hydrates can form from water and a gas, characterized in that said additive formation is incorporated into said fluid before hydrate formation, said additive formulation comprising at least two additives at least one of which behaves as a hydrate crystal nucleation agent.
In the formulations of the present invention, the nucleation agent(s) is (are) generally associated with one or more additives which can control the growth of hydrate crystals (growth inhibitors) and optionally one or more additives which can control their agglomeration (dispersing agents).
One preferred implementation of the invention consists of combining the nucleation agent with both a growth inhibitor and a hydrate crystal-dispersing agent. Such a formulation enables many nuclei to form; the growth and agglomeration of which are greatly retarded and the characteristics of which are such that they do not perturb transport of the fluid containing them.
The nucleation agents present in the formulations of the invention can be any water-soluble compound which can reduce the hydrate formation time. More particularly, it can be a water-soluble compound possessing one or more hydroxyl groups, such as alcohols or polyols. It may also be a water-soluble polymer containing one or more hydroxyl groups per monomer unit or a copolymer wherein at least one of the monomers constituting them possesses one or more hydroxyl groups. Examples which can be cited are polyvinyl alcohol, partially hydrolyzed poly (vinyl acetate), polyglycerols, alkylene glycol poly(meth)acrylates or poly methyl(meth)acrylates rendered water-soluble by aminolysis. The molecular mass of such polymers and copolymers are preferably relatively low, for example less than 50,000 and, in a preferred mode of the invention, less than 1,000. The nucleation agents also include quaternary phosphonium salts.
It should be noted that certain of these nucleation agents used in high concentration, for example over 10% by weight with respect to the water, behave as thermodynamic inhibitors. Further, certain polymer or copolymer type nucleation agents of the present invention can lose their specific nucleation agent characteristic if their molecular mass is too high.
A second constituent which can be present in the additive formulations of the invention can consist of a growth inhibitor. It can be any water-soluble polymer compound which can reduce the gas hydrate growth rate. It may, for example, be a polymeric compound already described in some of the Applicant""s patents, such as associative water-soluble copolymers described in U.S. Pat. No. 5,817,898 or water-soluble copolymers containing no heterocycle but containing at least one sulfonate group described in U.S. Pat. No. 5,789,635 or cationic polymers and copolymers described in French Patent No. 2 748 773. The molecular mass of growth inhibitors used in the formulation of the invention will be relatively high, preferably more than 50,000.
A third constituent which may be present in the additive formulations of the invention can consist of a dispersing agent which can prevent agglomeration of the hydrate crystals once they have formed. As an example, it can be selected from non-ionic, anionic, cationic or amphoteric compounds. Such low molecular weight compounds, for example less than 10,000, preferably 5000, will have a hydrophilic-lipophilic balance (HLB) such that they will be soluble or at least dispersible in water. Dispersing additives which can be used as the third constituent in formulations of the invention include compositions which a non-ionic amphiphilic nature such as polyethylene glycol polyisobutenylsuccinates.
It is also possible to cite compositions with a non-ionic amphiphilic nature obtained by reacting at least one unsaturated vegetable oil polymerized with an aminoalcohol. The aminoalcohol can be monoethanolamine or diethanolamine. Such compositions have been described by the Applicant, in particular in European Patent application No. 905 125. The use of these compositions as a hydrate dispersing additives has been described in European Patent application No. 905 350. The descriptions of these patent documents are hereby incorporated into the present description by reference.
Preferred additive formulations of the invention contain both at least one nucleation agent, at least one growth inhibitor and at least one hydrate crystal dispersing agent, each in a proportion of 5% to 50%, for example.
The process of the invention is aimed at improving the transport of an oilfield effluent comprising water, a gas and possibly a liquid hydrocarbon phase under thermodynamic conditions where hydrates can form from the water and the gas, comprising incorporating an additive formulation as described above into said effluent.
The additive formulation of the invention is generally added to an oilfield effluent to be treated in a concentration of 0.05% to 5% by weight, preferably 0.2% to 2% by weight, with respect to the water content of said effluent.
More particularly, the nucleation agent content introduced is at most 2% by weight, preferably at most 0.5% by weight, the amount of growth inhibitor is at most 2% by weight, preferably at most 1% by weight, and the amount of dispersing agent is at most 1% by weight, preferably at most 0.5% by weight with respect to the water present in the oilfield effluent to be treated.